In previous studies we have shown that fibronectin is a component of all serum cryoglobulin and synovial fluid cryoproteins tested. We also demonstrated that fibronectin co-purified with the complement component Clq from serum. Additional data has shown that fibronectin binds directly to Clq adsorbed to a solid phase and that the binding was inhibitable by poly-sulfated molecules and by heat aggregated IgG. Based upon our previous data and results from other laboratories, this proposal will study: (I) the interaction between fibronectin and Clq with respect to the stoichiometry and kinetic parameters of the binding; (II) the binding characteristics of fibronectin to preformed defined antibody-antigen complexes in the presence of Clq; (III) the sequence of interaction of fibronectin and the Cl-inhibitor with the isolated Cl complex, and with the Cl complex binding to and activated by antibody-antigen complexes; (IV) the biologicval roles of the fibronectin complement - immune complex interactions in vivo by determining circulating fibronectin-Clq and fibronectin-antibody-antigen complexes in an antigen induced immune complex system. Also, the effects of the fibronectin molecule on the quantitation of immune complexes in serum by the Clq-binding assay will be assessed. The binding interactions of fibronectin, Clq and Cl, with each other and with antibody-antigen complexes, will be determined using the radioiodinated molecules, and an anti-hapten - hapten-gel system. Activation of Cl will be quantitated by conversion of radiolabeled Cls in reconstituted Cl to Cls. The results of these studies will define the fibronectin interactions with Clq and immune complexes and delineate fibronectin's possible effects on their function and metabolism.